The over-all objective is to discover the processes whereby input from peripheral receptor tissue determines the morphology of and internal organization of developing sensory centers in the brain. We have established that the cuneate-gracile nuclear complex in pouch-young opossums is a suitable model system for such studies: the nuclear complex develops entirely after birth and is dependent upon peripheral input for its normal organization and development. The studies here proposed will determine the organization of synaptic connections within the nuclear complex, and the time and course of their development, using electron microscopic examination of normal material and the results of anterograde degeneration following selected lesions of afferents to the nuclear region. The normal morphology and development of the neuronal processes of this region will be analyzed using Golgi stains. The course of cell migration from ependymal germinal layer to adult nuclear morphology will be followed using H3-thymidine autoradiography. Finally we shall analyze the effects of removal of receptor tissue upon these aspects of nuclear ontogeny: synapse formation, growth of processes, cell migration, and cell degeneration and death using these same techniques (electron microscopy, Golgi staining and autoradiography). BIBLIOGRAPHIC REFERENCES: Weller, W. L. and J. I. Johnson. 1975 Barrels in cerebral cortex altered by receptor disruption in newborn, but not in five-day-old mice (Cricetidae and Muridae). Brain Research 83: 504-508. Johnson, J. I., E. W. Rubel and G. I. Hatton. 1974 Mechanosensory projections to cerebral cortex of sheep. Journal of Comparative Neurology 158:81-108.